An a priori language (from Latina priori - from the former) is any constructed language whose vocabulary is not based on existing languages, unlike a posteriori constructed languages.

Examples of famous a priori languages include Ro, Solresol, Mirad, Klingon, and Na'vi. By contrast, a posteriori languages are ones whose vocabulary is based on existing languages, either as a variation of one language (e.g., Latino sine flexione) or as a mixture of various languages.

Some a priori languages are designed to be international auxiliary languages that remove what could be considered an unfair learning advantage for native speakers of a source language that would otherwise exist for a posteriori languages. Some a priori languages try to categorize their vocabulary, either to express an underlying philosophy or to make it easier to recognize new vocabulary. These are also known as philosophical or taxonomic languages.